What is the default normalization in plot of stft function in Short-time Fourier transform

I was using matlab to plot Short-time Fourier transform (STFT).

My code was as follows:

[y,Fs] = audioread('p226_001.wav');

a = downsample(y,3);

stft(a,16000,'Window',hanning(512),'OverlapLength',128,'FFTLength',512);

It returned the following plot

The I plotted the origional values using the following code

[y,Fs] = audioread('p226_001.wav');

a = downsample(y,3);

[s, f, t] = stft(a,16000,'Window',hanning(512),'OverlapLength',128,'FFTLength',512);

surf(t, f, abs(s), 'EdgeColor', 'none');view(0,90);

Plot in this case was as follows

It is clear that some normalization has been done on the values to make plot brighter.

I was wondering that what exact normalization has been done for the plot of stft?

Answer 1

[jibrahim]

Hi Muhammad,

I beleive the plot you get with STFT uses log, so 20*log10(abs(S)+eps) is plotted.

[Muhammad Fazeel]

Thanks.